Promise.prototype.catch = function(onRejected){
    /**方法返回一个Promise (en-US)，并且处理拒绝的情况。
     * 它的行为与调用Promise.prototype.then(undefined, onRejected) 相同
     * (事实上, calling obj.catch(onRejected) 就是内部calls obj.then(undefined, onRejected)).
     * 
     * */
    return this.then(undefined,onRejected)
}